System and method for controlling a power budget at a power source equipment using a PHY

ABSTRACT

A system and method for controlling the delivery of power to a powered device in a Power over Ethernet Broad Reach (PoE-BR) application. Cabling power loss in a PoE-BR application is related to the resistance of the cable itself. A PHY can be designed to measure electrical characteristics (e.g., insertion loss, cross talk, length, etc.) of the Ethernet cable to enable determination of the cable resistance. The determined resistance in a broad reach cable can be used in increasing a power budget allocated to a power source equipment port.

BACKGROUND

1. Field of the Invention

The present invention relates generally to Power over Ethernet (PoE)systems and methods and, more particularly, to the control of powerdelivered to a powered device.

2. Introduction

The IEEE 802.3af PoE standard provides a framework for delivery of powerfrom power source equipment (PSE) to a powered device (PD) over Ethernetcabling. In this PoE process, a valid device detection is firstperformed. This detection process identifies whether or not it isconnected to a valid device to ensure that power is not applied tonon-PoE capable devices.

After a valid PD is discovered, the PSE can optionally perform a powerclassification. IEEE 802.3af defines five power classes for a PD device.The completion of this power classification process enables the PSE tomanage the power that is delivered to the various PDs connected to thePSE. If a particular power class is identified for a particular PD, thenthe PSE can allocate the appropriate power for that PD. If powerclassification is not performed, then a default classification can beused where the PSE supplies the full 15.4 W of power onto the particularport.

Management of the power budgets that are allocated to the various PDsconnected to the PSE is crucial for efficient operation of the PSE.Management of power budgets are even more critical in a PoE Broad Reach(PoE-BR) application where the PD is connected to the PSE using anEthernet cable greater than 100 meters (e.g., 300-500 meters). Ingeneral, the total amount of power that can be allocated to the variousPDs is limited by the capacity of the PSE. Thus, what is needed is amechanism that enables the PSE to identify an accurate amount of powerthat should be budgeted on each port.

SUMMARY

A system and/or method for controlling power delivered to powereddevices, substantially as shown in and/or described in connection withat least one of the figures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features of the invention can be obtained, a moreparticular description of the invention briefly described above will berendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 illustrates an embodiment of a Power over Ethernet (PoE) system.

FIGS. 2A and 2B illustrate circuit diagrams that model the PoE system.

FIG. 3 illustrates a flowchart of a PoE process.

FIG. 4 illustrates an embodiment of a PoE system that enablescommunication of cable characteristic information from a PHY to a PSE.

FIG. 5 illustrates a flowchart of a process for communicating cablecharacteristic information from a PHY to a PSE.

DETAILED DESCRIPTION

Various embodiments of the invention are discussed in detail below.While specific implementations are discussed, it should be understoodthat this is done for illustration purposes only. A person skilled inthe relevant art will recognize that other components and configurationsmay be used without parting from the spirit and scope of the invention.

FIG. 1 illustrates an embodiment of a power over Ethernet (PoE) system.As illustrated, the PoE system includes power source equipment (PSE) 120that transmits power to powered device (PD) 140. Power delivered by thePSE to the PD is provided through the application of a voltage acrossthe center taps of transformers that are coupled to a transmit (TX) pairand a receive (RX) pair of wires carried within an Ethernet cable. Thetwo TX and RX pairs enable data communication between Ethernet PHYs 110and 130.

As is further illustrated in FIG. 1, PD 140 includes 802.3af module 142.This module includes the electronics that would enable PD 140 tocommunicate with PSE 120 in accordance with the IEEE 802.3af standard.PD 140 also includes pulse width modulation (PWM) DC:DC controller 144that controls power FET 146, which in turn provides constant power toload 150. In general, there are two types of loads: a purely resistiveload (e.g., lamp) and a constant power load that is fed by a DC:DC powercontroller. The present application is primarily directed to constantpower loads fed by a DC:DC power controller.

The delivery of power from PSE 120 to load 150 can be modeled by thecircuit model illustrated in FIG. 2A. As illustrated, a power sourceprovides a voltage V_(PSE) to a circuit that includes a first parallelpair of resistors (R₁, R₂), a load resistance R_(LOAD), and a secondparallel pair of resistors (R₃, R₄). Here, the first parallel pair ofresistors R₁, R₂ represents the resistances of the TX pair of wires,while the second parallel pair of resistors R₃, R₄ represents theresistances of the RX pair of wires.

The values of resistors R₁, R₂, R₃, and R₄ depend on the type and lengthof Ethernet cable. Specifically, the resistors R₁, R₂, R₃, and R₄ have acertain resistance/length that is dependent on a type of Ethernet cable(e.g., Category 3, 5, 6, etc.). For example, for Category 5 Ethernetcable, resistors R₁, R₂, R₃, and R₄ would have a resistance ofapproximately 0.1Ω/meter. Thus, for a 100-meter Category 5 Ethernetcable, each of resistors R₁, R₂, R₃, and R₄ would have a value of 10Ω.In this example, parallel resistors R₁ and R₂ would have an equivalentresistance of 5Ω, while parallel resistors R₃ and R₄ would also have anequivalent resistance of 5Ω. In combination, the total value of theEthernet cable resistance (R_(cable)) can then be determined as the sumof 5Ω+5Ω=10Ω. A simplified PoE circuit model that includes the singlecable resistance value R_(cable) is illustrated in FIG. 2B. As notedabove, the resistance R_(cable) for Category 5 cable is approximately0.1Ω/meter. For 100 meters of Category 5 cable, the resistance R_(cable)is therefore 10Ω.

In the IEEE 802.3af standard, a PSE can optionally perform aclassification step that identifies a power classification of the PD.Table 1 below shows the five PD classes supported by the IEEE 802.3afstandard.

TABLE 1 Min Power Max Power Class Usage Output by PSE Input at PD 0Default 15.4 W 0.44 to 12.95 W 1 Optional  4.0 W  0.44 to 3.84 W 2Optional  7.0 W  3.84 to 6.49 W 3 Optional 15.4 W 6.49 to 12.95 W 4Reserved Act as Class 0 Reserved

As illustrated, the Class 0 (default) and Class 3 PD classificationsspecify the PSE's minimum output power as 15.4 W. For lower power PDssuch as Class 1 and Class 2 devices, the PSE's minimum output power isspecified as 4.0 W and 7.0 W, respectively. While optional, theidentification of the correct PD power classification enables the PSE tobudget only as much power as is needed on each port. This effectivelyincreases the capacity of the PSE in supplying power to a set ofconnected PDs.

It is a feature of the present invention that the measurement of one ormore characteristics of the Ethernet cable can be used to impact theoperation of the PoE system. In one embodiment, the measuredcharacteristics are used to identify a type and length of Ethernetcable. The identified type and length of the Ethernet cable can then beused to estimate the resistance of the Ethernet cable. In turn, theestimated resistance of the Ethernet cable can be used to assess powerlosses in the cable, which impacts the power budget that is allocatedfor a particular PSE port.

To illustrate this general process of the present invention, referenceis made to the flowchart of FIG. 3. As illustrated, the process beginsat step 302, where one or more characteristics of an Ethernet cable aremeasured. In one embodiment, this measurement step can be implemented aspart of the PHY's analysis of the electrical characteristics of theEthernet cable. For example, the measurement step can be implemented aspart of an echo cancellation convergence process implemented by the PHY.

In one embodiment, the one or more characteristics of the Ethernet cablethat are measured at step 302 are those characteristics that wouldenable the PoE system to better estimate the resistance of the Ethernetcable. Here, the estimate of the actual cable resistance would enablethe PoE system to estimate the actual power loss of the cable. In oneembodiment, the PHY is designed to measure characteristics that wouldenable a determination of the insertion loss, cross talk, and length ofthe Ethernet cable.

At step 304, after the one or more characteristics of the Ethernet cableare measured, the PoE system would then determine a type and length ofEthernet cable. The Ethernet cable type can be determined based on themeasured insertion loss, cross talk, and length of the Ethernet cable.These measurements of the Ethernet cable would enable the PoE system todetermine, for example, whether the Ethernet cable is a Category 3, 5,6, or 7 Ethernet cable.

As would be appreciated, the different cable types have differentresistances associated therewith. As noted, Category 3 Ethernet cablehas a resistance of approximately 0.2%/meter, while Category 5 Ethernetcable has a resistance of approximately 0.1Ω/meter. Once the type andlength of Ethernet cable is identified at step 304, the PoE system canthen determine its impact on the PoE system at step 306.

As will be described in greater detail below, the particular impact onthe PoE system can vary depending on the application. Here, it is afeature of the present invention that the cable type and length can beused by the PoE system in a dynamic configuration or operation process.For example, the cable type and length can be used to determine anadjustment to a power budget for a given PSE port.

In a PoE-BR application, the PD can be connected to the PSE with morethan 100 m of Ethernet cable. For example, a PoE-BR application can bedefined to support distances up to 500 m or beyond. In this environment,the cable type and length can have a significant impact on the PoE-BRsystem.

In the circuit model of FIG. 2B, where the PD includes a DC:DCconverter, the load R_(L) would receive constant power, P_(L), and see avoltage V_(L) on its input. Since P_(L) is fixed at the load,P_(L)=I*V_(L), where I is the current going through the whole circuit.The power loss of the cable would then be P_(loss)=I²*R_(cable).

In specifying the minimum output power of 15.4 W for the PSE, the IEEE802.3af standard assumes a worst-case link resistance of 20Ω when the PDis connected to the PSE using 100 m of Category 3 cable. At a currentlimit of 350 mA, the worst-case power loss attributed to the cable isP_(loss)=(350 mA)²*20Ω=2.45 W. This worst-case power loss of 2.45 W isthe difference between the PSE's minimum output power and the max powerdrawn by the PD (i.e., 15.4 W−12.95 W=2.45 W).

In general, the increase in distance between the PSE and PD (e.g., 500 mand beyond) creates a greater range of potential operation in a PoE-BRsystem. This range of operation makes it increasingly difficult toprovide system specifications using worst-case operating parameters. Forexample, assume that up to 500 m of Category 3 cable is supported by thePoE-BR specification. In addressing this scenario, the resistance of thecable would have a range of 20Ω-100Ω. If the 100Ω worst-case cableresistance is assumed then it would be impractical in identifying powerbudgets such as that listed in Table 1.

Specifically, a worst-case resistance of 100Ω would lead to a worst-casecable power loss of P_(loss)=(350 mA)²*100Ω=12.25 W. This worst-casecable power loss would then require that 12.95 W+12.25 W=25.2 W beallocated to each port that has a Class 3 or Class 0 Default PDclassification.

It is therefore a feature of the present invention that the power budgetallocated to a PoE-BR PSE port can be dynamically changed based on ananalysis of the Ethernet cabling coupled to that port. In oneembodiment, the characteristics of the Ethernet cabling is used todynamically increase a specified power budget based on thecharacteristics of the cable.

To illustrate this feature of the present invention assume that a PoE-BRPSE would budget 15.4 W for a port that is connected to a Class 3 PD.Here, it should be noted that while the PoE-BR PSE's power budgets wouldlikely be increased to accommodate the broad reach application, it hasbeen kept the same as standard PoE power budgets for illustrationpurposes.

In this context, assume that it is determined that a PD is connected via200 m of Category 5 cabling. In this case, the resistance of the cablewould be approximately 20Ω. Across the cable, the voltage drop can bedefined as V_(PSE)−V_(L)=I*R_(cable). This equation can be solved forthe voltage V_(L) allowed at the PD as follows:V _(PSE) −V _(L) =I*R _(cable)V _(PSE) −V _(L)=(P _(L) /V _(L))*R _(cable)V _(PSE) *V _(L) −V _(L) ² =P _(L) *R _(cable)V _(L) ² −V _(PSE) *V _(L) +P _(L) *R _(cable)=0V _(L) =[V _(PSE) +/−SQRT(V _(PSE) ²−(4*P _(L) *R _(cable)))]/2

If V_(PSE) is known to be 50V, P_(L) is 12.95 W (max power allowed forClass 3 PD), and R_(cable)=20Ω, thenV_(L)=(50+/−SQRT(50²−4*12.95*20))/2=(50+/−38.26)/2=44.13V. The currentcan then be calculated using V_(PSE)−V_(L)=I*R_(cable), such that50V−44.13V=I*20Ω results in I=0.294 A. The total power output by the PSEis then 12.95 W plus the power loss in the cable. The power loss in thecable in this case is I²*R_(cable)=(0.294 A)²*20Ω=1.73 W. The totalpower budget attributed to the PSE port in this example would be 12.95W+1.73 W=14.68 W. In this example, the total power output by the PSE iswithin the 15.4 W power budget, so no adjustment would be necessary.

If, on the other hand, it is determined that the PSE is connected to thePD via 400 m of Category 5 cable, then the cable resistance would beapproximately 40Ω. The voltage at the PD would then be calculated asV_(L)=(50+/−SQRT(50²−4*12.95*40))/2=(50+/−20.69)/2=35.34V. The currentcan then be calculated using V_(PSE)−V_(L)=I*R_(cable), such that50V−35.34V=I*40Ω results in I=0.366 A. The total power output by the PSEis then 12.95 W plus the power loss in the cable. The power loss in thecable in this case is I²*R_(cable)=(0.366 A)²*40Ω=5.36 W. The totalpower budget attributed to the PSE port in this example would then be12.95 W+5.36 W=18.31 W. In this example, the total power output by thePSE is greater than the 15.4 W power budget. Here, an adjustment wouldthen be made at the PSE. Specifically, the power budget allocated tothat port would be dynamically increased to accommodate the 18.31 Woutput power. In one embodiment, this dynamic change could be effectedthrough a new current limit on that port.

In another example, assume that P_(L) is 12.95 W, R_(cable) isdetermined to be 60Ω (300 m of Category 3 cable), and V_(L) is known tobe 40V. As would be appreciated, V_(L) can be communicated from the PDto the PSE using various communication means, such as some form of layer2 communication. In this case, the current I can be calculated usingI=P_(L)/V_(L)=12.95 W/40V=0.323 A. In this case, the estimated powerloss of the cable is I²*R_(cable)=(0.323 A)²*60Ω=6.26 W, which can thenbe used to estimate the total power budget of 12.95 W+6.26 W=19.21 W.Here again, an adjustment would be made at the PSE, wherein the powerbudget allocated to that port would be dynamically increased toaccommodate the 19.21 W output power.

As demonstrated, the power budget attributable to the port can varywidely due to the range of distances being served by the PoE-BRapplication. To accommodate such a range of distances, and hence cablepower losses, a dynamic adjustment mechanism is provided that ensuresthat a power budget on a port is increased only when it is needed. Thisis in sharp contrast to conventional systems that provide a worst-casepower budget allocation to a port, thereby ensuring that unused powercapacity is unnecessarily kept in reserve.

As noted above, one or more characteristics of the Ethernet cable aremeasured to enable the PoE system to estimate the resistance of theEthernet cable, and ultimately to estimate the actual power loss of theEthernet cable. To facilitate such an estimate, the PoE system canmeasure such characteristics as the insertion loss, cross talk, length,etc. of the Ethernet cable. The measurement of the insertion loss, crosstalk, and length of the Ethernet cable represents one example of thecharacteristics that can be used to estimate the cable resistance, andhence the power loss in the cable.

In one embodiment, cable length can be determined directly using TDR. Inan alternative embodiment, cable length can be determined indirectlybased on data generated in the measurement of insertion loss using around trip of the injected signal. Here, the time interval betweenlaunching and receiving the pulse(s) is linearly proportional to thecable length. The cable length can then be computed by multiplying thepropagation speed with the time interval, then divided by two to accountfor the round-trip delay.

As has been described, various cable characteristics can be used todetermine a cable type and length. These factors enable a determinationof the resistance and power loss of the cable. As would be appreciated,other characteristics beyond those described above could also be used toenable the PoE system to determine the resistance and power loss of thecable. Regardless of the measurement data that is used, it issignificant that the PoE system can use the data to adjust some aspectof configuration or operation of the PoE system dynamically.

FIG. 4 illustrates an embodiment of a PoE environment 400 in which theprinciples of the present invention can be implemented. As illustrated,environment 400 includes PHYs 430-1 to 430-N that are each connected toEthernet switch 420. While a PHY can include one or more Ethernettransceivers, the wiring for only a single transceiver is illustrated asbeing connected to PHY 430-N. Each PHY is also connected to CPU 410,although only a single connection from CPU 410 to PHY 430-N is shown forsimplicity. In one embodiment, CPU 410 is incorporated along withEthernet switch 420 and PHYs 410-1 to 410-N on a single chip. In anotherembodiment, Ethernet switch 420 and PHYs 410-1 to 410-N are incorporatedon a single chip separate from CPU 410, wherein communication with CPU410 is enabled via a serial interface. Also illustrated in PoEenvironment 400 is a PSE 440 that provides power through the center tapsof the transformers shown. As illustrated, PSE 440 is also coupled toCPU 410. In one embodiment, PSE 440 is coupled to CPU 410 viaopto-isolator 450 that facilitates an isolation boundary.

To illustrate the operation of PoE environment 400 in implementing theprinciples of the present invention, reference is now made to theflowchart of FIG. 5. As illustrated, the flowchart of FIG. 5 begins atstep 502 where a transceiver in PHY 430-N measures line characteristicsof an Ethernet cable coupled to PHY 430-N. In one embodiment,measurements that enable a determination of insertion loss, cross talk,and cable length are taken during an echo canceller convergence processperformed by an echo canceller module under control of CPU 410. Linecharacteristic measurements taken by the transceiver are thentransmitted to CPU 410 at step 504.

Next, at step 506, CPU 410 uses the line characteristic measurement datato determine the cable type and cable length. This cable type and lengthinformation is subsequently provided to PSE 440 at step 508. Here, itshould be noted that PSE can also be configured to determine the cabletype and length using the line characteristic measurement data.

Regardless of where the cable type and length is determined, itsavailability to PSE 440 would enable PSE 440 to determine its impact onthe PoE system configuration and/or operation. This impact determinationcan consider the cable type and length, and hence resistance of thecable, in combination with other PoE system parameters such as V_(PSE),P_(L), V_(L), etc. As would be appreciated, the impact analysis can beperformed by any system element that is responsible for diagnosing theEthernet cable, determining an adjustment to a power budget for a givenPSE port, etc. In general, the impact analysis can be based on one ormore parameters such as the cable link resistance, cable current,V_(PSE), P_(L), V_(L), that can either be communicated, discovered, orassumed by the appropriate system element. For example, one or moreparameters can be based on a system specification, derived through oneor more calculations using measurement data (e.g., cable resistancederived from determined cable type and length), or received from anothersystem element with knowledge of such a parameter (e.g., V_(L)communicated to the PSE by the PD).

These and other aspects of the present invention will become apparent tothose skilled in the art by a review of the preceding detaileddescription. Although a number of salient features of the presentinvention have been described above, the invention is capable of otherembodiments and of being practiced and carried out in various ways thatwould be apparent to one of ordinary skill in the art after reading thedisclosed invention, therefore the above description should not beconsidered to be exclusive of these other embodiments. Also, it is to beunderstood that the phraseology and terminology employed herein are forthe purposes of description and should not be regarded as limiting.

1. A power over Ethernet system that allocates individual power budgetsfor each of a plurality of powered devices that are coupled to a powersource equipment via a corresponding plurality of power source equipmentports, comprising: a first cable detection component that measures oneor more electrical characteristics of a first Ethernet cable thatcouples a first of said plurality of powered devices to a first of saidplurality of power source equipment ports, said first Ethernet cablehaving a length that is less than 100 meters; a second cable detectioncomponent that measures one or more electrical characteristics of asecond Ethernet cable that couples a second of said plurality of powereddevices to a second of said plurality of power source equipment ports,said second Ethernet cable having a length that is between 300 and 500meters; and a power controller that controls an allocation of a totalpower budget amongst said plurality of power source equipment ports byan assignment of an individual power budget to each of said plurality ofpower source equipment ports, said allocation of said total power budgetbeing based on a first individual power budget that accounts for a firstpower loss determined for said first Ethernet cable having a length lessthan 100 meters using said one or more measured electricalcharacteristics of said first Ethernet cables, and on a secondindividual power budget that accounts for a second power loss determinedfor said second Ethernet cable having a length between 300 and 500meters using said one or more measured electrical characteristics ofsaid second Ethernet cable.
 2. The power over Ethernet system of claim1, wherein said individual power budgets are determined using anEthernet cable type.
 3. The power over Ethernet system of claim 2,wherein said individual power budgets are determined using an Ethernetcable length.
 4. The power over Ethernet system of claim 1, wherein saidpower controller controls an allocation of power based on a calculatedEthernet cable resistances.
 5. The power over Ethernet system of claim1, wherein said power controller identifies power budgets allocated toeach of said plurality of power source equipment ports.
 6. The powerover Ethernet system of claim 1, wherein a power allocation is changedthrough an adjustment to a current limit.
 7. A power over Ethernetmethod in a system that supplies power to a plurality of powered devicesvia a plurality of ports in a power source equipment, comprising: uponconnection of a first powered device to a first power source equipmentport via a first Ethernet cable that is less than 100 meters, measuringone or more electrical characteristics of said first Ethernet cable;determining a first individual power budget for said first power sourceequipment port using said one or more electrical measurements of saidfirst Ethernet cable; upon connection of a second powered device to asecond power source equipment port via a second Ethernet cable that isbetween 300 and 500 meters, measuring one or more electricalcharacteristics of said second Ethernet cable; determining, prior to anallocation of power to said second power source equipment port, a secondindividual power budget for said second power source equipment portusing said one or more electrical measurements of said second Ethernetcable, wherein said second individual power budget is greater than 15.4W and accounts for a cable power loss that is attributable to saidsecond Ethernet cable that is between 300 and 500 meters; controlling anassignment of portions of a total power budget available to said powersourcing equipment to said plurality of power source equipment ports,wherein said control equipment port and an assignment of said secondindividual power budget to said second power source equipment port; andpowering said first powered device over said first Ethernet cable thatis less than 100 meters and said second powered device over said secondEthernet cable that is between 300 and 500 meters.
 8. The method ofclaim 7, wherein said determining said second individual power budget isbased on an identified length of said second Ethernet cable.
 9. Themethod of claim 7, wherein said determining said second individual powerbudget is based on an identified type of said second Ethernet cable. 10.The method of claim 7, wherein said determining said second individualpower budget is based on a determined resistance of said second Ethernetcable.
 11. The method of claim 7, wherein said controlling comprisessetting a current limit for a power source equipment port.